sedimentary
Chert Concretion
Sedimentary Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Creamy white to light tan, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Creamy white to light tan, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica within sedimentary rock layers, often limestone or chalk. The nodular or irregular shape occurs as silica replaces organic matter or fills voids during diagenesis.
Uses & applications
Primarily of interest to collectors; historically used for stone tools (flint) due to its sharp edges when broken.
Geological facts
Cylindrical or irregular nodes like this are often mistaken for fossils (pseudofossils) due to their organic-looking shapes. Chert is extremely chemically stable and resistant to weathering.
Field identification & locations
Identify by its hardness (it will scratch glass) and waxy texture. Commonly found in creek beds or weathered out of carbonate rock outcrops. Collectors look for unique shapes or 'pocket' inclusions.
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